1. Field of the Invention
The invention relates generally to the field of semiconductor device manufacturing and, more particularly, to methods for forming metal nitride films by a combination of atomic layer deposition (ALD) processes for depositing metal nitride and elemental metal. For example, smooth TixWyNz films may be formed by utilizing atomic layer deposition processes for depositing TiN and W.
2. Description of the Related Art
Atomic layer deposition (ALD) is based on sequential, self-saturating surface reactions, which can provide good conformality and step coverage regardless of the geometry of the structure to be coated. However, deposition of metallic films by ALD has been challenging, in part because ALD is based essentially on thermodynamically favorable half-reactions. Apart from the noble metals, elements in their pure forms are not the thermodynamically most stable forms of the elements but rather their compounds. Therefore, the choice of precursors for depositing metallic films by ALD has been a challenging task.
Metallic compound films such as nitrides and carbides are easier to deposit by ALD compared to pure elemental films. However, the thermodynamic stability of these films is typically also substantially lower than their corresponding metal oxides and the same difficulty in the precursor chemistry choice arises as with the elemental films.
Refractory metal conducting layers are basic building blocks in micro and nano-electronics. Titanium nitride and tungsten layers are commonly used in the semiconductor manufacturing industry. Titanium nitride is used, for example, as a gate electrode material or as a copper diffusion barrier. Tungsten is mainly used as the contact plug material in metal 1 level interconnects. Both materials can be deposited by physical vapor deposition (PVD), by chemical vapor deposition (CVD) or by ALD methods. For ultra-high aspect ratio structures found in the current state of the art microelectronic chips and in future nodes, ALD deposition methods are preferred because they are capable of providing better conformality and step coverage.
In addition to the electrical properties of the metal films, such as resistivity and work function, one of the most important properties of the films is their microstructure. Metallic films favor a polycrystalline phase, often having specific grain morphology. During the deposition process, many metals adopt columnar grain morphology with a certain preferred crystal orientation relative to the substrate. The grain boundaries between the columnar grains present discontinuities in the material, which alter the mechanical and electrical properties of the films and may serve as diffusion channels for impurities. As a result, the desired material properties are degraded compared to their amorphous or single crystal phases. In the case of nanocrystalline alloys however, the grain boundaries may also have a positive effect on the material properties, namely by thermodynamically stabilizing the nanocrystalline phase of the alloy through the segregation of the elemental distributions of the alloy elements between the grain boundaries and the bulk of the grains.